Encapsulated enzymes with integrated fluorescence-control of enzymatic activity

A fluorescence-based particle sensor for oxaloacetic acid is presented. In the presence of nicotinamide adenine dinucleotide as a cofactor, oxaloacetic acid is converted by malate dehydrogenase into l-malic acid. The progress of the reaction is monitored by sensing of proton consumption with an integrated pH sensor. The kinetics of this sensor are investigated on a single particle level. This work demonstrates the feasibility to detect analytes upon their enzymatic conversion into a product, which in turn can be sensed with a fluorophore responding to changes in the concentration of this product. Integration of enzymes and fluorophores into one carrier particle, as demonstrated here for the case of polyelectrolyte polymer capsules, allows the range of analytes that can be detected with fluorescence to be extended, as it enhances selectivity. This coupled system allows enzymatic activity, as well as the kinetics of malate dehydrogenase, to be monitored.